HEATLOSSPRORadiator output at heat pump temperatures
A radiator's catalogue output assumes boiler temperatures. At the 35–50 °C a heat pump runs at, it gives much less. Here is how much, and how to check whether yours are big enough.
In short: output = rated output at ΔT 50 × (ΔT ÷ 50)1.3, where ΔT is the mean water temperature minus the room temperature. At 45 °C flow, 40 °C return and 21 °C in the room (ΔT 21.5), a radiator gives about 33% of its rated output, so a 1,500 W radiator delivers about 500 W.
Radiator calculator
Correction factors by flow temperature
For a room at 21 °C and a return 5 °C below the flow. Multiply the ΔT 50 output by the factor.
| Flow / return | Mean water | ΔT | Factor | 1,000 W radiator gives | In BTU/h |
|---|---|---|---|---|---|
| 75 / 65 °C | 70 °C | 49 K | 0.97 | 974 W | 3,324 |
| 70 / 60 °C | 65 °C | 44 K | 0.85 | 847 W | 2,890 |
| 55 / 50 °C | 52.5 °C | 31.5 K | 0.55 | 548 W | 1,871 |
| 50 / 45 °C | 47.5 °C | 26.5 K | 0.44 | 438 W | 1,495 |
| 45 / 40 °C | 42.5 °C | 21.5 K | 0.33 | 334 W | 1,139 |
| 40 / 35 °C | 37.5 °C | 16.5 K | 0.24 | 237 W | 807 |
| 35 / 30 °C | 32.5 °C | 11.5 K | 0.15 | 148 W | 505 |
At 35 °C flow a radiator gives about a seventh of its rating. That is why many heat pump systems run radiators at 45–50 °C, or use underfloor heating at 35 °C.
How to check your radiators for a heat pump
- Work out each room's heat loss at the design outdoor temperature. See how to calculate heat loss.
- Find each radiator's rated output from its type and size (type 11, 21, 22 or 33, height and length) in the manufacturer's catalogue.
- Choose a design flow temperature, for example 45 °C, and correct each output with the factor above.
- Compare. Where the corrected output is below the room's heat loss, fit a bigger radiator, add one, or plan a slightly higher flow temperature for the whole house.
The HeatLoss Pro designer does all four at once. Draw the rooms and place the radiators, and every room shows its heat loss and its radiators' output at the heat pump's flow temperature, with the shortfalls marked. It also sizes the pipes and balances the circuits. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
How do I calculate radiator output at a lower flow temperature?
Radiators are rated at ΔT 50: the mean water temperature 50 °C above the room. Work out your own ΔT (the average of the flow and return temperatures, minus the room temperature) and multiply the rated output by (ΔT ÷ 50) to the power 1.3. At 45 °C flow, 40 °C return and a 21 °C room, ΔT is 21.5 and a radiator gives about a third of its rated output.
Do I need bigger radiators for a heat pump?
Often in some rooms, not all. A boiler runs radiators at 60–75 °C; a heat pump is most efficient at 35–50 °C, where a radiator gives a third to a half of its rating. Compare each radiator's output at the heat pump's flow temperature with its room's heat loss: only rooms where it falls short need a bigger or an extra radiator.
What flow temperature should a heat pump run at?
As low as the radiators allow. Every degree lower raises the efficiency by roughly 2–3%. Many systems are designed for 45 °C at the coldest outdoor temperature, or 35 °C with underfloor heating, and run lower in milder weather with weather compensation.
What is ΔT 50 on a radiator?
It is the test condition behind the catalogue output (EN 442): water averaging 70 °C in a 20 °C room, a difference of 50 K. Older catalogues sometimes quote ΔT 60, which gives a higher number for the same radiator; convert it first by multiplying by (50 ÷ 60)^1.3, about 0.79.
How do I convert radiator BTU to watts?
Divide BTU per hour by 3.412 to get watts, or multiply watts by 3.412 to get BTU/h. A 5,000 BTU/h radiator is about 1,465 W at ΔT 50. Then apply the same correction for the lower heat pump temperature: at 45 °C flow it gives about a third of that.
What does a type 22 radiator mean?
The first digit is the number of panels and the second the number of convector fin sets: type 11 is one panel with fins, type 21 two panels with one set, type 22 two panels with two sets, and type 33 three of each. More panels and fins give more output from the same wall space, which helps at heat pump temperatures.
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